Organisms avoid mutation by replacing bases that do not correctly match the complementary bases in DNA double strands. Delay in the modification of new strands during DNA replication, or the presence of nicks, may permit discrimination of the old (correct) strand from the new. There are repair systems for the removal of bases damaged by radiation or chemical treatment. A new mechanism for the correction of mismatched bases in specific DNA sequences has been identified in E. coli. This very short patch (VSP) correction system is responsible for the recovery of excess cI+ recombinants in crosses with specific mutations in the bacteriophage lambda repressor (cI) gene. Mismatched bases in DNA heteroduplexes accompanying recombination are replaced by a process that removes less than 20 adjoining bases. The repairprone mutations are CtoT changes in sequences 5-CC-AGG, C-AGG and CC-AG. VSP repair maintains a C.G base pair at the indicated site. Among mutations in genes known to affect DNA repair, only mutS reduces VSP repair. Mutations at known sites in gamma gene N will be crossed to confirm the predicted correlation of excess N+ recombinants with amber mutations in 5-CCAG sequences. By directed mutagenesis, a new 5-CCAG sequence will be produced in gene cI and crossed with a mutation that produces the sequence CTAG. Bacterial mutations that abrogate VSP repair will be isolated by transposon mutagenesis followed by mapping and cloning. DNA repair leading to restoration of an EcoRII site (CCAGG) in heterduplexes of phiX174 DNA will be studied in vitro. Other bacterial genera will be screened for in vitro VSP repair activity.